A device of this type and a method of this type are known from DE 100 43 601 A1. This document describes a circular-symmetrical device for depositing in particular III-V semiconductor layers on III-V semiconductor substrates. The known device has a circular-cylindrical process chamber extending in the horizontal plane. The floor of the process chamber is formed by a heated substrate holder. On the substrate holder there are a multiplicity of substrate carriers in circular arrangement around the center of the holder. One or more substrates may be disposed on each of the substrate carriers. The substrate carriers are rotationally driven. The ceiling of the process chamber, opposite the floor of the process chamber, may likewise be heated. At the center of the ceiling there is a gas inlet member. This protrudes into the process chamber. The portion of the gas inlet member that protrudes into the process chamber is water-cooled. The gas inlet member forms two gas inlet zones, lying vertically one above the other. The inlet zone that is disposed immediately above the floor is located between the floor plate and an end face of the gas inlet member, which has at its center an opening from which a hydride emerges together with a carrier gas. This hydride may be arsine, phosphine or ammonia. Above this inlet zone there is a further inlet zone, through which a gaseous starting material, likewise mixed in a carrier gas, is introduced into the process chamber. This gaseous starting material may be TMGa, TMIn or some other metalorganic compound.
Under typical process conditions, the stream of the first starting material, which flows through the gas inlet zone neighboring the floor of the process chamber, is considerably greater than that which flows through the second gas inlet zone. The starting material which flows through the first gas inlet zone is also of a considerably higher concentration than the starting material which flows through the second gas inlet zone, so that not only is the velocity of the gas flowing through the first gas inlet zone considerably greater than the velocity of the gas flowing through the second gas inlet zone, but the densities of the gases also differ considerably.
In an inlet zone directly adjoining the gas inlet member, the starting materials are partially thermally decomposed. In this zone, a homogenization of the flow or a homogenization of the gas phase also takes place. The two starting materials must mix with each other. In a growing zone adjoining the inlet zone downstream are the substrates. In this zone, the gas phase concentration of the reactants, and in particular of the III component, decreases as the distance from the gas inlet member increases. This gas phase depletion is accompanied by a fall in the growth rate as the distance from the gas inlet member increases. To make the growth more uniform, compensation is provided by the rotation of the substrate carriers. These circumstances are described by DE 100 57 134 A1.
The position of the limit between inlet zone and growing zone is determined by the maximum of the growth rate. This maximum lies where the pre-decomposition or homogenization of the gas phase and of the stream is substantially completed and the growth-limiting group III starting materials are diffused by the dense highly concentrated stream of gas from the lower inlet. The maximum is intended to lie shortly before the beginning of the growing zone in the direction of flow.
If it is wished to increase the capacity of the known device, allowing more substrates to be coated simultaneously, the extent of the growing zone must be increased. At the same time, however, the supply of starting materials must also be increased. If the stream of gas of the starting materials into the process chamber is increased, the limit between inlet zone and growing zone shifts away from the gas inlet member. However, increasing the gas inlet zone in this way is undesired, since undesired adducts can form in this zone. On the other hand, however, the maximum of the growth rate must not lie within the growing zone, in order to ensure homogeneous growth of the layers on the substrates. In addition, increasing the inlet zone entails either a reduction of the growing zone or a structural increase in the size of the entire process chamber. The latter is undesired for reasons of cost.
It is consequently an object of the invention to specify measures as to how the useful surface area in a process chamber can be increased. This increase is at the same time to be possible without reducing the packing density of the substrates on the substrate holder.